Image pickup lens system

ABSTRACT

An image pickup lens system includes a lens  1  of a meniscus shape having a first face as a convex  concave face, and a diaphragm  2  disposed on an object side of the lens  1.  The lens  1  meets the following conditions: Y′/fl equal to or larger than 0.6; Dt/Dc is equal to or smaller than 0.9 and equal to or larger than 0.5; and Ap 2 /Am 2  is equal to or larger than 0.9. Thus, even when an image pickup element is of a reduced size, a desired optical performance can be maintained, and it is possible to produce a lens easily with a back focal length ensured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup lens system andparticularly, to an image pickup lens system of a single lensconstruction, which is used in an image pickup device (e.g., a CCDcamera) utilizing an image pickup element such as CCD, CMOS and the likemounted in a mobile telephone or the like, and which can be reduced inweight and size.

2. Description of the Related Art

In recent years, the multimedia have been developed remarkably, and forexample, the demand for a camera utilizing an image pickup element suchas CCD, CMOS and the like adapted to be mounted in a portable computer,a visual telephone, a mobile telephone and the like, e.g., a CCD camera,is being increased remarkably. Such a CCD camera is required to bemounted in a limited space and for this reason, it is desired that theCCD camera is small in size and lightweight.

Therefore, it is also desired that an image pickup lens system used insuch a CCD camera is likewise small in size and lightweight.

A single-lens system using a single lens is conventionally used as suchan image pickup lens system.

There is such a conventionally known image-pickup lens systems of asingle lens type disclosed, for example, in Japanese Patent ApplicationLaid-open No.10-282410 and the like.

A ¼ inch sensor (having a diagonal dimension of about 4.5 mm) has beenconventionally used in many cases as a solid image pickup elementmounted in a mobile telephone. In recent years, however, a 1/7 inchsensor (having a diagonal dimension of about 2.6 mm) of a furtherreduced size has been used mainly in place of the above-described ¼ inchsensor.

In the image pickup lens system of such an extremely small size, it isnecessary to meet the following conditions:

First of all, in the image pickup element such as CCD, CMOS and thelike, 100% of light obliquely entering into the image pickup elementcannot be utilized for the structural reason, unlike a silver-salt filmand hence, it is required that light enters into a lens at an anglecloser to the vertical at any position on the image pickup element. Inother words, it is required that a telecentric property is high, and adistance between the image pickup element and a pupil is large.

In the image pickup element, light is sensed by each of picture elementsprovided in the image pickup element, but the sensitiveness of eachpicture element within the image pickup element is uniform and hence, itis desirable that the amount of light entering into the image pickupelement is kept constant at any position within the image pickupelement. In other words, it is required that the amount of light isuniform in a central portion and a peripheral portion of the imagepickup element and hence, it is required that the amount of light in theperipheral portion ensured as much as possible.

These conditions are more important than those in the prior art, becausethe size of each picture element is reduced with a reduction in size ofthe image pickup element and as a result, the sensitiveness is alsoreduced.

Further, in general, a cover glass is disposed on a surface of the imagepickup element, and depending on the application, any one of variousfilters (such as an IR cutting filter, a low-band pass filter and thelike) is also inserted. Therefore, a relatively long back focal lengthis required.

For example, in an image pickup lens of a wide-angle type used in the ¼inch sensor, it is relatively easy to ensure a distance between theimage pickup element and a pupil and a back focal length, because thefocal length is as relatively long as about 4 mm.

In an image pickup lens of a wide-angle type used in the 1/7 inchsensor, however, the focal length is as extremely short as about 2 mm.For this reason, the distance between the image pickup element and apupil and the back focal length are also shortened with a reduction infocal length. Therefore, the following problem is encountered: It isextremely difficult that the image pickup lens used in the conventional¼ inch sensor is reduced in size and used for the /7 inch sensor. Whenthe size of the lens is merely reduced, as described above, there isalso a problem that the accuracies of the thickness of the centralportion of the lens and the thickness of a flange are degradedremarkably.

The position of the pupil can be set at the maximum by disposing adiaphragm in the vicinity of the position of the focal length on theside of an object. In this case, however, the following problem isencountered: The size of the entire lens (the thickness and theeffective diameter of an optical face) is increased extremely, but alsothe workability of the lens is degraded.

Further, the back focal length can be improved by forming the lens intoa meniscus shape, but there is a problem that if the lens is not of anappropriate shape, the workability of the lens is degraded.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animage pickup lens system, wherein even when the image pickup element isof a reduced size, a desired optical performance can be maintained, andthe lens can be produced easily with a back focal length ensured.

To achieve the above object, according to a first aspect and feature ofthe present invention, there is provided an image pickup lens systemcomprising a lens of a meniscus shape having a first face as a convexconcave face located on the object side, and a diaphragm disposed atobject side of the lens, the lens meeting the following conditions:Y′/fl≧0.6  (1)0.9≧Dt/Dc≧0.5  (2)1.0≧Ap₂/Am₂≧0.9  (3)wherein fl is a focal length of the entire lens system;

-   -   Y′ is a maximum image height    -   Dt is a thickness of a thinnest portion of the lens in an area        including at least one optical face;    -   Dc is a thickness of a central portion of the lens;    -   Ap₂ is an effective radius of a second face of the lens on the        side of the image surface (a maximum radius of a portion through        which effective light rays pass); and    -   Am₂ is a maximum radius of the second face on the side of the        image surface.

With the first feature of the present invention, a large angle of viewcan be ensured by setting Y′/fl in the expression (1) at 0.6 or more. Inaddition, if Dt/Dc in the expression (2) is larger than 0.9, therefraction force of the lens is insufficient and hence, a desired focallength can not be realized, band in addition, the correction of variousaberrations is insufficient. If Dt/Dc is smaller than 0.5, an unbalanceduniformity of the lens thickness is increased and as a result, theformability of the lens is degraded, and it is difficult to work theoptical face. Further, the second face of the lens body on the side ofthe image surface can be utilized effectively by setting Ap₂/Am₂ in theexpression (3) at 0.9 or more (the maximum value of Ap₂/Am₂ is 1.0). Inaddition, the radius of the optical face can be suppressed to achieve areduction in size of the lens, and the optical face formed by workingcan be utilized without any waste, whereby an amount of light in theperipheral portion of the lens can be ensured and hence, an amount oflight in the peripheral portion of the lens can be ensured.

In the present invention, the thickness of the central portion of thelens and the thickness of the peripheral portion of the lens are set ina special relationship by meeting the conditions represented by theexpressions (1), (2) and (3). Thus, in a state in which the producibleshape of the lens has been maintained, the amount of light in theperipheral portion of the lens can be ensured, and even when the imagepickup element is of a reduced size, the performance of the lens can bemaintained.

According to a second aspect and feature of the present invention, inaddition to the first feature, the lens body meets the followingconditions:0.15≧S≧0.03  (4)wherein S is a distance (mm) between the diaphragm and the first face ofthe lens.

With the second feature, if S in the expression (4) is larger than 0.15mm, the thickness of the entire optical system is increased and inaddition, the effective diameter of the second face of the lens isincreased. For this reason, it is difficult to reduce the size of thelens, and the formability and workability of the lens are degraded. If Sis smaller than 0.03 mm, the position of a pupil is too short, and theangle of incidence of light into a sensor surface is increased, and forthis reason, it is difficult to effectively utilize light.

In the present invention, a telecentric property can be ensured bydisposing the diaphragm so as to meet the expression (4) and hence, evenwhen the image pickup element is of a reduced size, a good image picturecan be obtained.

According to a third aspect and feature of the present invention, inaddition to the first or second feature, the lens body meets thefollowing conditions:0.55≧|r₂/fl|≧0.35  (5)0.8≧Dc/fl≧0.3  (6)wherein r2 is a radius of curvature of a central portion of the face onthe side of the image surface;

-   -   fl is a focal length of the entire lens system; and    -   Dc is a thickness of a central portion of the lens.

With the third feature, if |r₂/fl| in the expression (5) is larger than0.55, a back focal length cannot be ensured, and various filters and thelike cannot be inserted. If |r₂/fl| is smaller than 0.35, it isdifficult to work the periphery of the optical face and in addition, theunbalanced uniformity of the lens thickness is increased. Thus, it isimpossible to form the lens with a good accuracy. If Dc/fl in theexpression (6) is larger than 0.8, the entire optical system isincreased in size. If Dc/fl is smaller than 0.3, the thickness of aflange for supporting the peripheral portion of the lens body isinsufficient, resulting in a degraded productivity.

In the present embodiment, the size of the lens can be reduced, and aback focal length can be ensured by meeting the expressions (5) and (6),and when the image pickup element is of a reduced size, the lens can beproduced easily.

The above and other objects, features and advantages of the inventionwill become apparent from the following description of the preferredembodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an arrangement of an embodiment ofan image pickup lens system according to the present invention;

FIG. 2 is a schematic illustration showing an arrangement of a firstexample of an image pickup lens system according to the presentinvention;

FIG. 3 is graphs showing the spherical aberration, the astigmatism andthe distortion of the image pickup lens system shown in FIG. 2;

FIG. 4 is a schematic illustration showing an arrangement of a secondexample of an image pickup lens system according to the presentinvention;

FIG. 5 is graphs showing the spherical aberration, the astigmatism andthe distortion of the image pickup lens system shown in FIG. 4;

FIG. 6 is a schematic illustration showing an arrangement of a thirdexample of an image pickup lens system according to the presentinvention; and

FIG. 7 is graphs showing the spherical aberration, the astigmatism andthe distortion of the image pickup lens system shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described by way of an embodimentshown in the accompanying drawings.

FIG. 1 shows the basic structure of an image pickup lens systemaccording to the present invention. This lens includes a lens body 1made of a resin such as a plastic. In the image pickup lens system inthe present embodiment, the meniscus-shaped lens 1 having a firstconcave surface located on the side of an object is disposed, and adiaphragm 2 is disposed at the object side of the lens 1. A light-amountlimiting plate 3 is disposed at a peripheral edge of a second face ofthe lens 1 on the side of an image surface, and a cover glass 4 and animage pickup surface 5 which is a light-receiving surface of an imagepickup element such as CCD or CMOS are disposed on the side of thesecond face of the lens 1.

In the present embodiment, the lens 1 is adapted to meet the followingconditions:Y′/fl≧0.6  (1)0.9≧Dt/Dc≧0.5  (2)1.0≧Ap₂/Am₂≧0.9  (3)wherein fl is a focal length of the entire lens system; Y′ is a maximumimage height (a length equal to ½ of a diagonal line in an image pickuparea on the image pickup surface 5); Dt is a thickness of a thinnestportion in an area including at least one optical face of the lens; Dcis a thickness of a central portion of the lens; Ap₂ is an effectiveradius of the optical face on the side of the image surface (a maximumradius of a portion through which effective light rays pass); and Am₂ isa maximum radius of the optical face on the side of the image surface (aradius of the optical face as worked).

A large angle of view can be ensured by setting Y′/fl in the expression(1) at 0.6 or more.

If Dt/Dc in the expression (2) is larger than 0.9, the refraction forceof the lens is insufficient and hence, a desired focal length can berealized, but also the correction of various aberrations isinsufficient. If Dt/Dc is smaller than 0.5, an unbalanced uniformity ofthe lens thickness is increased and as a result, the formability of thelens is degraded, and it is difficult to work the optical face. It ismore preferable that Dt/Dc is equal to or larger than 0.7.

Further, the second face of the lens 1 on the side of the image surfacecan be utilized effectively by setting Ap₂/Am₂ in the expression (3) at0.9 or more (the maximum value of Ap₂/Am₂ is 1.0). In addition, theradius of the optical face can be suppressed to achieve a reduction insize of the lens, and the optical face formed by working can be utilizedwithout any waste, whereby an amount of light in the peripheral portionof the lens can be ensured.

In the present embodiment, the thickness of the central portion of thelens and the thickness of the peripheral portion of the lens are set ina special relationship by meeting the conditions represented by theexpressions (1), (2) and (3). Thus, in a state in which the producibleshape of the lens has been maintained, the amount of light in theperipheral portion of the lens can be ensured, and even when the imagepickup element is of a reduced size, the performance of the lens can bemaintained.

Preferably, the lens 1 is formed to meet the following condition:0.15≧S≧0.03  (4)wherein S is a distance (mm) between the diaphragm 2 and the first faceof the lens 1.

If S in the expression (4) is larger than 0.15 mm, the thickness of theentire optical system is increased and in addition, the effectivediameter of the second face of the lens 1 is increased. For this reason,it is difficult to reduce the size of the lens 1, and the formabilityand workability of the lens 1 are degraded. If S is smaller than 0.03mm, the position of a pupil is too short, and the angle of incidenceinto a sensor surface is increased. For this reason, it is difficult toeffectively utilize light, resulting in a reduction in amount of lightin the peripheral portion of the lens.

In the present embodiment, a telecentric property can be ensured bydisposing the diaphragm so as to meet the expression (4) and hence, evenwhen the image pickup element is of a reduced size, a good image picturecan be obtained.

Further, preferably, the lens body 1 is formed to meet the followingconditions:0.55≧|r₂/fl|≧0.35  (5)0.8≧Dc/fl≧0.3  (6)wherein r₂ is a radius of curvature of the central portion of thesurface on the side of the image surface.

If |r₂/fl| in the expression (5) is larger than 0.55, a back focallength cannot be ensured, and various filters and the like cannot beinserted. If |r₂/fl| is smaller than 0.35, it is difficult to work theperiphery of the optical face, and the unbalanced uniformity of the lensthickness is increased. Thus, it is impossible to form the lens with agood accuracy.

If Dc/fl in the expression (6) is larger than 0.8, the entire opticalsystem is increased in size (increased both in a direction of an opticalaxis and in a diametrical direction) If Dc/fl is smaller than 0.3, thethickness of a flange for supporting the peripheral portion of the lensbody is insufficient, resulting in a degraded productivity.

In the present embodiment, the size of the lens can be reduced, and aback focal length can be ensured by meeting the expressions (5) and (6),and when the image pickup element is of a reduced size, the lens can beproduced easily.

EXAMPLES

Examples of the present invention will be described with reference toFIGS. 2 to 7.

In the examples, F represents an F number; fl represents a focal lengthof the entire lens system; Y′ represents a maximum image height; Dtrepresents a thickness of a thinnest portion in an area including atleast one optical face of a lens; Dc represents a thickness of thecentral portion of the lens; Ap₂ represents an effective radius of theoptical face on the side of an image surface (a maximum radius of aportion through which effective light rays pass; Am₂ represents amaximum radius of the optical face on the image surface (a radius of theoptical face as worked; S represents a distance between a diaphragm 2and a first face of a lens 1; r₂ represents a radius of curvature of thecentral portion of a second face of the lens 1; r represents a radius ofcurvature of each of the optical faces; d represents a lens thickness oran air gap, and nd represents a refractive index.

If a Z axis is taken in a direction of an optical axis; an X axis istaken in a direction perpendicular to the optical axis, and a directionof travel of light is defined to be positive, the aspherical shape ofthe lens is represented by the following equation:$Z = {\frac{\frac{x^{2}}{r}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{x^{2}}{r^{2}}}}} + {ax}^{4} + {bx}^{6} + {cx}^{8}}$wherein each of k, a, b and c is an aspherical factor.

Example 1

FIG. 2 shows a first example of the present invention. This example isan image pickup lens system having an arrangement shown in FIG. 1. Theimage pickup lens system in this example is set under the followingconditions:

-   -   F=2.8; fl=2.19 mm; Y′=1.35 mm; Dc=0.90 mm; Dt=0.70 mm; Am₂=0.65        mm; Ap₂=0.62 mm; S=0.05 mm; and r₂=0.99 mm.

Radius Refractive Abbe constant Face r of curvature Distance d index ndνd (1) Diaphragm 0.000 0.0500 (2) First face −5.153 0.9000 1.52 56.0 oflens (3) Second face −0.989 0.0000 of lens (4) First face 0.000 0.70001.52 64.2 of cover glass (5) Second face 0.000 1.8597 of cover glass (6)CCD face K A B 2 3.450423e + 001 −3.596798e − 001   1.975480e + 000 31.339285e − 001   5.011488e − 002 −5.827157e − 002 c 2 −8.831343e + 0003   7.319106e − 003

Under such conditions, Y′/fl=1.35/2.19=0.616 is established to meet theexpression (1).

In addition, Dt/Dc=0.70/0.90=0.78 is established to meet the expression(2).

Further, Ap₂/Am₂=0.62/0.65=0.95 is established to meet the expression(3).

Yet further, S=0.05 is established to meet the expression (4).

Yet further, |r₂/fl|=0.99/2.19=0.452 is established to meet theexpression (5).

Yet further, Dc/fl=0.90/2.19=0.411 is established to meet the expression(6).

The spherical aberration, the astigmatism and the distortion in theimage pickup lens system of the example 1 are shown in FIG. 3.

As can be seen from FIG. 3, all of the spherical aberration, theastigmatism and the distortion assume substantially satisfactory values,and hence, sufficient optical characteristics can be provided.

Example 2

FIG. 4 shows a second example of the present invention. The image pickuplens system is set under the following conditions:

-   -   F=2.0; fl=1.67 mm; Y′=1.05 mm; Dc=0.85 mm; Dt=0.75 mm; Am₂=0.55        mm; Ap₂=0.50 mm; S=0.10 mm; and r₂=0.85 mm.

Radius Refractive Abbe constant Face r of curvature Distance d index ndνd (1) Diaphragm 0.000 0.1000 (2) First face −30.971 0.8500 1.52 56.0 oflens (3) Second face −0.851 0.0000 of lens (4) First face 0.000 0.70001.52 64.2 of cover glass (5) Second face 0.000 1.2213 of cover glass (6)CCD face K a B 2   0.000000e + 000 −4.680585e − 001   7.754132e − 001 3−1.339285e − 001 −1.199533e − 001 −1.180505e − 001 C 2 −5.542358e + 0003 −4.081729e − 003

Under such conditions, Y′/fl=1.05/1.67=0.629 is established to meet theexpression (1).

In addition, Dt/Dc=0.75/0.85=0.88 is established to meet the expression(2).

Further, Ap₂/Am₂=0.50/0.55=0.91 is established to meet the expression(3).

Yet further, S=0.10 is established to meet the expression (4).

Yet further, |r₂/fl|=0.85/1.67=0.509 is established to meet theexpression (5).

Yet further, Dc/fl=0.85/1.67=0.509 is established to meet the expression(6).

The spherical aberration, the astigmatism and the distortion in theimage pickup lens system of the example 2 are shown in FIG. 5.

As can be seen from FIG. 5, all of the spherical aberration, theastigmatism and the distortion assume substantially satisfactory values,and hence, sufficient optical characteristics can be provided.

Example 3

FIG. 6 shows a third example of the present invention. The image pickuplens system is set under the following conditions:

-   -   F=2.0; fl=2.20 mm; Y′=1.35 mm; Dc=1.00 mm; Dt=0.84 mm; Am₂=0.70        mm; Ap₂=0.68 mm; S=0.10 mm; and r₂=1.07 mm.

Radius Refractive Abbe constant Face r of curvature Distance d index ndνd (1) Diaphragm 0.000 0.1000 (2) First face −10.950 1.0000 1.52 56.0 oflens (3) Second face −1.068 0.0000 of lens (4) First face 0.000 0.70001.52 64.2 of cover glass (5) Second face 0.000 1.8054 of cover glass (6)CCD face K A B 2 0.000000e + 000 −2.586316e − 001 3.667821e − 001 32.268728e − 001   4.688898e − 002 2.073563e − 002 C 2 −1.422459e + 000 3  0.000000e + 000

Under such conditions, Y′/fl=1.35/2.20=0.614 is established to meet theexpression (1).

In addition, Dt/Dc=0.84/1.00=0.84 is established to meet the expression(2).

Further, Ap₂/Am₂=0.68/0.70=0.97 is established to meet the expression(3).

Yet further, S=0.10 is established to meet the expression (4).

Yet further, |r₂/fl|=1.07/2.20=0.486 is established to meet theexpression (5).

Yet further, Dc/fl=1.00/2.20=0.455 is established to meet the expression(6).

The spherical aberration, the astigmatism and the distortion in theimage pickup lens system of the example 3 are shown in FIG. 7.

As can be seen from FIG. 7, all of the spherical aberration, theastigmatism and the distortion assume substantially satisfactory values,and hence, sufficient optical characteristics can be provided.

Although the embodiment of the present invention has been described indetail, it will be understood that the present invention is not limitedto the above-described embodiment, and various modifications may be madeas required without departing from the spirit and scope of the inventiondefined in claims. For example, the light-amount limiting plate disposedon the side of the image surface is necessarily not required, and thepresent invention is also applicable to an arrangement in which thelight-amount limiting plate is not mounted.

1. An image pickup lens system comprising a lens of a meniscus shapehaving a first face as a convex concave face located on the object side,and a diaphragm disposed at the object side of the lens, said lensmeeting the following conditions:Y′/fl≧0.6  (1)0.9≧Dt/Dc≧0.5  (2)1.0≧Ap₂/Am₂≧0.9  (3) wherein fl is a focal length of the entire lenssystem; Y′ is a maximum image height; Dt is a thickness of a thinnestportion of the lens in an area including at least one optical face; Dcis a thickness of a central portion of the lens; Ap₂ is an effectiveradius of a second face of the lens on the side of the image surface (amaximum radius of a portion through which effective light rays pass);and Am₂ is a maximum radius of the second face on the side of the imagesurface.
 2. An image pickup lens system according to claim 1, whereinsaid lens meets the following conditions:0.15≧S≧0.03  (4) wherein S is a distance (mm) between said diaphragm andthe first face of said lens.
 3. An image pickup lens system according toclaim 1 or 2, wherein said lens body meets the following conditions:0.55≧|r₂/fl|≧0.35  (5)0.8≧Dc/fl≧0.3  (6) wherein r2 is a radius of curvature of a centralportion of the face on the side of the image surface; fl is a focallength of the entire lens system; and Dc is a thickness of a centralportion of the lens.